Mechanisms for the Influence of Citrus Rootstocks on Fruit Size

To obtain insight into potential mechanisms underlying the influence of rootstock on fruit size, we performed a comparative analysis of 'Shatangju' mandarin grafted onto two rootstocks. The results demonstrated that trees grafted onto Canton lemon produced larger fruits through an enhancement of cell expansion in the ripening period. The difference in fruit size may be due to greater auxin levels in fruits from trees on Canton lemon, and different auxin levels may be produced by parent trees as the result of AUX1 upregulation. Rootstocks also modulate auxin signaling by affecting the transcription of several auxin response factor genes. There were higher abscisic acid concentrations in fruits of 'Shatangju'/Trifoliate orange, resulting in an inhibition of fruit growth and cell expansion through suppression of the synthesis of growth promoting hormones. Furthermore, expansins may be involved in the regulation of final fruit size by influencing cell expansion. Multiple pathways likely exist in citrus rootstocks that regulate fruit size.